1. Field of the Invention
The present invention relates to the field of inversion for supplying alternating voltage to electric motors, in particular electric motors in electric vehicle drives.
2. Description of the Related Art
Electric vehicle drives ordinarily include synchronous machines excited by a permanent magnet, to which alternating voltage is supplied using an inverter. FIG. 1 shows an inverter system for operating an electric motor 101 using an inverter circuit, also referred to as an intermediate circuit, which is designed as a B6 bridge. The inverter circuit includes upper half-bridge branches having high-voltage-side switching elements 103, 105 and 107 and lower half-bridge branches having low-voltage-side switching elements 109, 111 and 113. In each case, a diode 115 is situated in series with each switching element 105 through 113. If transistor switches are used to implement the switching elements, diodes 115 and switching elements 103 through 113 are ordinarily implemented jointly by one transistor switch. Freewheeling diodes 117 are situated in each case in parallel with the particular series connection made up of one diode 115 and one switching element 103 through 113 each. Furthermore, an intermediate circuit capacitor 119 is connected in parallel with the inverter circuit, intermediate circuit capacitor 119 being connected during operation in parallel with a constant voltage source which is connected to an input port 121, 123 of the inverter system. As a pulse-controlled inverter, the inverter system represented in FIG. 1 generates a three-phase alternating voltage for activating electric motor 101 with the aid of the B6 bridge inverter circuit.
If an error is detected in the system represented in FIG. 1, electric motor 101 is switched into an active short-circuit by closing three lower switches 109 through 113 or three upper switches 103 through 107. In this case, electric motor 101 behaves electrically neutral, i.e., electric motor 101 neither draws electrical power from the intermediate circuit nor does it emit electrical power to the intermediate circuit. Mechanically, electric motor 101 generates a short-circuit torque on the shaft, corresponding to the power loss in the ohmic resistances of the windings.
However, different situations are conceivable in which switching of electric motor 101 into the active short-circuit is not possible, is technically complex, is not practical or is not even allowed. Such a case occurs, for example, when the short-circuit torque is large. Furthermore, a hardware effect may occur, with the result that it may no longer be possible to switch into the active short-circuit or do so only with great technical complexity, for example, if the circuit breakers have been blown or are no longer closeable, which does not allow an active short-circuit. Furthermore, it may be the case that the circuit breakers are no longer openable, or that the voltage supply of the pulse-controlled inverter is defective, so that the power section of the pulse-controlled inverter must be supplied from a high-voltage source. Furthermore, it may be the case that electric motor 101 is not sustained short-circuit-proof.
One alternative to the active short-circuit in the error case is also for electric motor 101 to freewheel, in which case all the circuit breakers are opened. In the case of freewheeling, the electric motor generates a speed-dependent voltage by induction. If the induced voltage is, for example, higher than the voltage applied to the intermediate circuit, it is fed back into the intermediate circuit via freewheeling diode 117. A negative torque corresponding to the recovered energy then acts upon the shaft of the electric motor. Moreover, the high-voltage battery connected to the pulse-controlled inverter on the input side may be overcharged and damaged due to the energy recovery.